Kotlin語言學習筆記（6）

運算符重載（Operator overloading）

一元運算符

Expression	Translated to
+a	a.unaryPlus()
-a	a.unaryMinus()
!a	a.not()

data class Point(val x: Int, val y: Int)
operator fun Point.unaryMinus() = Point(-x, -y)
val point = Point(10, 20)
println(-point)  // prints "(-10, -20)"

遞增遞減運算符

Expression	Translated to
a++	a.inc() + see below
a--	a.dec() + see below

算術運算符

Expression	Translated to
a + b	a.plus(b)
a - b	a.minus(b)
a * b	a.times(b)
a / b	a.div(b)
a % b	a.rem(b), a.mod(b) (deprecated)
a..b	a.rangeTo(b)

data class Counter(val dayIndex: Int) {
    operator fun plus(increment: Int): Counter {
        return Counter(dayIndex + increment)
    }
}

in 運算符

Expression	Translated to
a in b	b.contains(a)
a !in b	!b.contains(a)

下標存取運算符

Expression	Translated to
a[i]	a.get(i)
a[i, j]	a.get(i, j)
a[i_1, ..., i_n]	a.get(i_1, ..., i_n)
a[i] = b	a.set(i, b)
a[i, j] = b	a.set(i, j, b)
a[i_1, ..., i_n] = b	a.set(i_1, ..., i_n, b)

調用運算符

Expression	Translated to
a()	a.invoke()
a(i)	a.invoke(i)
a(i, j)	a.invoke(i, j)
a(i_1, ..., i_n)	a.invoke(i_1, ..., i_n)

加強運算符

Expression	Translated to
a += b	a.plusAssign(b)
a -= b	a.minusAssign(b)
a *= b	a.timesAssign(b)
a /= b	a.divAssign(b)
a %= b	a.remAssign(b), a.modAssign(b) (deprecated)

相等不等運算符

Expression	Translated to
a == b	a?.equals(b) ?: (b === null)
a != b	!(a?.equals(b) ?: (b === null))

比較運算符

Expression	Translated to
a > b	a.compareTo(b) > 0
a < b	a.compareTo(b) < 0
a >= b	a.compareTo(b) >= 0
a <= b	a.compareTo(b) <= 0

Null 安全性

// 可空類型和非空類型
var a: String = "abc"
a = null // compilation error
var b: String? = "abc"
b = null // ok
val l = a.length
val l = b.length // error: variable 'b' can be null
// 檢查 null
val l = if (b != null) b.length else -1
if (b != null && b.length > 0) {
    print("String of length ${b.length}")
} else {
    print("Empty string")
}
// 安全調用
b?.length
bob?.department?.head?.name
val listWithNulls: List<String?> = listOf("A", null)
for (item in listWithNulls) {
     item?.let { println(it) } // prints A and ignores null
}
// If either `person` or `person.department` is null, the function is not called:
person?.department?.head = managersPool.getManager()
// Elvis運算符
val l: Int = if (b != null) b.length else -1
val l = b?.length ?: -1
fun foo(node: Node): String? {
    val parent = node.getParent() ?: return null
    val name = node.getName() ?: throw IllegalArgumentException("name expected")
    // ...
}
// !! 運算符
val l = b!!.length
// 安全轉型
val aInt: Int? = a as? Int
// 可空類型的集合類型
val nullableList: List<Int?> = listOf(1, 2, null, 4)
val intList: List<Int> = nullableList.filterNotNull()

異常（Exceptions）

Kotlin 語言不提供 checked exception

// throw表達式 和 try表達式
throw MyException("Hi There!")
try {
    // some code
}
catch (e: SomeException) {
    // handler
}
finally {
    // optional finally block
}
// try 是表達式
val a: Int? = try { parseInt(input) } catch (e: NumberFormatException) { null }
// throw 表達式的類型是 Nothing
val s = person.name ?: throw IllegalArgumentException("Name required")
val s = person.name ?: fail("Name required")
println(s)     // 's' is known to be initialized at this point
val x = null           // 'x' has type `Nothing?`
val l = listOf(null)   // 'l' has type `List<Nothing?>

註解（Annotations）

註解的參數類型包括：

• Java的基本類型
• 字符串
• 枚舉
• 其餘註解
• 以上類型的數組

使用端的註解包括：

• file
• property（Java不可見）
• field
• get（getter）
• set（setter）
• receiver（擴展函數以及擴展屬性的接收者參數）
• param（主體構造器的參數）
• setparam（setter的參數）
• delegate（委託屬性）

// 語法
annotation class Fancy
// 詳細語法
@Target(AnnotationTarget.CLASS, AnnotationTarget.FUNCTION,
        AnnotationTarget.VALUE_PARAMETER, AnnotationTarget.EXPRESSION)
@Retention(AnnotationRetention.SOURCE)
@MustBeDocumented
annotation class Fancy
// 用法
@Fancy class Foo {
    @Fancy fun baz(@Fancy foo: Int): Int {
        return (@Fancy 1)
    }
}
// 給主體構造器加註解時必須使用constructor關鍵字
class Foo @Inject constructor(dependency: MyDependency) {
    // ...
}
// 給屬性存取器加註解
class Foo {
    var x: MyDependency? = null
        @Inject set
}
// 帶參數的註解
annotation class Special(val why: String)
@Special("example") class Foo {}
// 把註解用做其餘註解的參數
annotation class ReplaceWith(val expression: String)
annotation class Deprecated(
        val message: String,
        val replaceWith: ReplaceWith = ReplaceWith(""))
@Deprecated("This function is deprecated, use === instead", ReplaceWith("this === other"))
// 把類用做註解的參數
import kotlin.reflect.KClass
annotation class Ann(val arg1: KClass<*>, val arg2: KClass<out Any>)
@Ann(String::class, Int::class) class MyClass
// 給lambda表達式加上註解
annotation class Suspendable
val f = @Suspendable { Fiber.sleep(10) }
// 使用端的註解
class Example(@field:Ann val foo,    // annotate Java field
              @get:Ann val bar,      // annotate Java getter
              @param:Ann val quux)   // annotate Java constructor parameter
// 給整個文件加註解
@file:JvmName("Foo")
package org.jetbrains.demo
// Java註解
import org.junit.Test
import org.junit.Assert.*
import org.junit.Rule
import org.junit.rules.*

class Tests {
    // apply @Rule annotation to property getter
    @get:Rule val tempFolder = TemporaryFolder()

    @Test fun simple() {
        val f = tempFolder.newFile()
        assertEquals(42, getTheAnswer())
    }
}
// ①
// Java
public @interface Ann {
    int intValue();
    String stringValue();
}
// Kotlin
@Ann(intValue = 1, stringValue = "abc") class C
// ①
// Java
public @interface AnnWithValue {
    String value();
}
// Kotlin
@AnnWithValue("abc") class C
// ②
// Java
public @interface AnnWithArrayValue {
    String[] value();
}
// Kotlin
@AnnWithArrayValue("abc", "foo", "bar") class C
// ③
// Java
public @interface AnnWithArrayMethod {
    String[] names();
}
// Kotlin 1.2+:
@AnnWithArrayMethod(names = ["abc", "foo", "bar"]) 
class C
// Older Kotlin versions:
@AnnWithArrayMethod(names = arrayOf("abc", "foo", "bar")) 
class D
// ④
// Java
public @interface Ann {
    int value();
}
// Kotlin
fun foo(ann: Ann) {
    val i = ann.value
}